Fine structure of maxillary sensilla of larval Toxorhynchites brevipalpis (Diptera: Culicidae) with comments on the role of sensilla in behavior

Each maxilla of fourth instar Toxorhynchites brevipalpis bears nine sensilla: Four are located at the tip of the maxillary palp and five on the maxillary body. At the palp tip are three tapered pegs on bulbous bases (MS₁, MS₂, MS₆) that are innervated by four, two, and two neurons, respectively, and probably function in chemoreception. Also at the palp tip is a sturdy, cuticular rod with a lumen (MS₅) that opens distally to the exterior. The proximal end of the rod is closed by a cuticular base to which a single unbranched dendrite containing only a few microtubules is attached. The function of MS₅ is enigmatic; possibilities include mechanoreception and detection of infrared radiation. On the maxillary body are two tapered pegs on a common bulbous base (GS₁, GS₂) that are each innervated by three neurons, and probably are chemosensory. Three setae also occur on the maxillary body. They arise from prominent sockets and are each innervated by a neuron terminating at the hair base as a tubular body, a characteristic of cuticular mechanosensilla. The maxillary sensilla are innervated by a total of 18 neurons: 14 are probably chemosensory, three mechanosensory, and one is of unknown function. These results, combined with those from a previous study on antennal sensilla (Jez and McIver, '80), indicate that the mechanosensitive neurons of the antennae and maxillae are a relatively small percentage of the total mechanosensilla on the entire larva. In contrast the chemosensitive neurons of the antennae and maxillae provide most of the information about the chemical environment of the larva. T. brevipalpis has three less than the maximum of seven maxillary palpal sensilla found in larval mosquitoes so far studied. This difference may reflect a lesser need for sensory information about the acceptability of potential food in predators compared to browsers and filter-feeders.     

Apical antennal sensilla in nymphs of Libellula depressa (Odonata: Libellulidae)

Abstract. In an ultrastructural study of the apical antenna of the last nymphal stages of Libellula depressa (Odonata: Libellulidae), we found long sensilla trichodea, 2 sensory pegs, and a coeloconic sensillum on the last article of the flagellum (the distal part of the antenna). The long sensilla trichodea are mechanoreceptors, almost identical to the long filiform hairs of some terrestrial insects and the first sensilla of this kind to be described in aquatic insects. Particular attention was given to the complex coeloconic sensillum, a compound sensillum innervated by 2 groups of 3 neurons wrapped in a dendritic sheath. A cuticular sleeve envelops the distal portion of the outer dendritic segment. The cuticle of the coeloconic sensillum shows wide channels and is contiguous to the underlying granular and fibrillar layer. Similar structures on the antennae of the adults of other dragonflies were identified as chemoreceptors in previous studies. We hypothesize that this larval coeloconic sensillum might likewise have a chemosensory function, responding to molecules that diffuse through the cuticle and the underlying granular and fibrillar layer, as no clear pore or pore-tubule system is visible. Alternative functions are also explored on the basis of morphological details.     

Digitiform Sensilla on the Maxillar Palp of Coleoptera

The fine structure of the digitiform sensilla on the distal segment of the maxillar palps of Tenebrio and Dermestes is described. Each sensillum is associated with a single sensory cell and three enveloping cells, which enclose two receptor lymph cavities. The inner receptor lymph cavity of both species shows a different structural feature. Branches of the outer dendritic segments, which contain numerous microtubules, run to the tip of the hairshaft. A dendritic sheath extends to the apex of the peg. The hairshaft possesses a second canal, which is free of dendrites. The poreless hairshaft is inserted in a cuticular canal; the longer distal part of the shaft is positioned in a narrow superficial groove. The digitiform sensilla do not show the typical structures of mechanosensitive sensilla. The absence of pores in the setal wall does not point to a function as olfactory or gustatory hairs. The presumed function of the sensilla is discussed in relation to thermo-, hygro- and CO₂-receptors.     

Structure of galeal sensory complex in adults of the red turnip beetle,Entomoscelis americana brown (Coleoptera,Chrysomelidae)

Summary The internal and external structure of the galeae of the adult red turnip beetle, Entomoscelis americana, was studied using SEM and TEM. The galea broadens from base to truncated tip and its sides are of thick, sculpted cuticle invested with pores and coarse spines. The tip is of thinner, flexible cuticle covered with 8–12 uniporous, blunt-tipped apical pegs and a single, aporous, sharply-pointed apical hair.The coarse spines are singly innervated probable mechanosensilla owing to the tubular body at the distal end of the dendrite. These sensilla likely act as tactile hairs monitoring galeal-effected movements of food particles into the functional mouth. The pores are associated with glands within the galea. The function of the presumed secretion is not known but may be to keep objects and dried saliva from sticking to the mouthparts.The apical pegs are innervated by five neurons, each producing a single dendrite. Four dendrites enter the single peg lumen and communicate with the terminal pore. The fifth differentiates into a tubular body that inserts into the peg base. These are typical insect contact chemosensilla that, because of their location, would taste incoming food.The apical hair has no pores but is innervated by two neurons, each extending a dendrite into the hair lumen in chemosensillar fashion. The sensory mode of this sensillum is unknown but is probably not mechanoor chemoreception. Many of its features, reminiscent of taste hairs, lead us to hypothesize that it represents a one-time chemosensillum recently modified to a new form and sensory mode.Because larval and adult E. americana share similar food plant requirements, we hypothesize that similarities will be seen in their mouthpart sensilla. Comparisons of the adults and larvae show the common features between their respective galeal taste hairs are only those of insect contact chemosensilla in general. However, the adult apical hair and the larval medial sensillum show striking specific structural similarities. We propose that these are true structural and functional homologues.     

Fine structure of tarsal sensory organs in the whip spider Admetus pumilio (Amblypygi, Arachnida).

The sensory organs on the tarsi of the antenniform first legs of the whip spider Admetus pumilio C. L. Koch (Amblypygi, Arachnida) were examined with the scanning and transmission electron microscope. At least four different types of hair sensilla were found: (1) thick-walled bristles, which have the characteristics of contact chemoreceptors (several chemoreceptive dendrites in the lumen plus two mechanoreceptors at the base); (2) short club sensilla, innervated by 4-6 neurons which terminate in a pore on the tip; they are possibly humidity receptors; (3) porous sensilla, which are either innervated by 20-25 neurons and have typical pore tubules, or they have 40-45 neurons but no pore tubules; both types are considered to be olfactory; (4) rod sensilla occur in clusters near segmental borders; they are innervated by only one large dendrite which branches inside the lumen. Other tarsal receptors are the claws, which correspond to contact chemoreceptors, and the pit organ which resembles the tarsal organ of spiders. Compared to other arthropod sensilla, the contact chemoreceptors are very similar to those of spiders, while the porous sensilla correspond structurally to olfactory receptors in insects; the club and rod sensilla seem to be typical for amblypygids.     

Innervation of the cercal sensilla on the ovipositor of the Australian sheep blowfly (Lucilia cuprina)

ABSTRACT. The ovipositor of the female sheep blowfly, Lucilia cuprina (Wied.) (Diptera: Calliphoridae), has a complement of cercal sensilla that includes long, medium and short tactile hairs, two campaniform domes, four olfactory pegs, and ten double-channelled gustatory hairs. This sensory array is suited to assess oviposition site resources, prior to and during the laying of an egg batch.
The tactile hairs and campaniform sensilla are each innervated by a single, tubular body containing dendrite. The olfactory pegs are each innervated by a single, moderately branched dendrite, which gains access to the external environment via pores at the bottom of deep grooves in the peg wall. The gustatory hairs fall into two categories. Four hairs have a single, tubular body containing dendrite at their base, and four unbranched dendrites running up to the hair tip which has a terminal pore. Six of the taste hairs have no tubular body containing dendrite at the base, and three unbranched dendrites running up to a terminal pore.     

Morphology and ultrastructure of sense organs on the ovipositor of Trybliographa rapae,a parasitoid of the cabbage root fly

The ovipositor of the parasitoid wasp Trybliographa rapae was examined by scanning and transmission electron microscopy. Characteristic peg-like sensilla with a cuticular ring at the base are found at the tip of the ventral valves, where they occur in a characteristic arrangement of triplets. The unusual basal structure probably protects the sensilla against damage during movement through the substrate and piercing of the host cuticle. These sensilla are each innervated by six dendrites, some of which have lamellated tips, generally considered to be characteristic of thermosensitivity. It is suggested that the remaining dendrites are gustatory, and as such probably respond to factors present in host haemolymph. A second type of peg-like sensillum is found on both the dorsal and the ventral valves. These are set in deep pits so that only the tip of the peg protrudes above the surface of the cuticle. These occur along the length of the ovipositor shaft and ultrastructural studies reveal the pegs to be innervated by a single mechanosensitive dendrite, probably monitoring the movement of the ovipositor through the substrate.     

Fine structure and role in behavior of sensilla on the terminalia of Aedes aegypti (L.) (Diptera: Culicidae)

The terminalia of male and female Aedes aegypti (L.) bear numerous hairs of various shapes and lengths, all of which are mechanoreceptors. Each hair is innervated by one bipolar neuron which contains ciliary rootlets, two basal bodies, and a region assuming the structure of a non-motile cilium. At the distal tip of the dendrite is a tubular body, a characteristic of cuticular mechanoreceptors. Covering the outer dendritic segment is a cuticular sheath which ends proximally in a net-like felt-work and distally attaches to the hair base. Each hair sensillum has two sheath cells. Presumed efferent fibers are associated with the sheath cells. On the insula of the female terminalia are a few campaniform sensilla, the domes of which are raised into small pegs. The sensilla on the terminalia function in copulation and oviposition and probably in warning. A sequence of neurological events is traced for copulation and oviposition. Other cuticular structures, viz., scales, microtrichia, acanthae, and aedeagal spines, which occur on the terminalia are not innervated.     

Labellar taste organs of Drosophila melanogaster

There are 36 to 42 taste bristles on each half of the labellum of Drosophila melanogaster; most of them are two-pronged with a pouch between them. Some end bluntly with a pore at the tip. Each taste-bristle has two lumina: one is circular, the other crescent-like in cross section. In most bristles four dendrites of chemoreceptor neurons run along the circular lumen. In five to seven taste-bristles only two chemoreceptor neurons are found. A mechanoreceptor neuron sends a dendrite to the base of each taste-bristle. The dendrites are surrounded by four concentrically-arranged sheath cells. The inner cell secretes the cuticular sheath; cells II and III are presumably two trichogens, one secreting the bristle material around the circular lumen, the other around the crescent-like lumen. Cell IV, especially rich in bundles of microtubules, secretes the cuticle of the socket, and corresponds to the tormogen. The neurons have the typical structure found in insect sensilla. In many sensilla one neuron is less electron-dense than the others and may be the water-sensor. On the medial side of the labellum between the pseudotracheae are rows of taste pegs covered by folds. In each peg one chemoreceptor and one mechanoreceptor are found. The number of axons in each labial nerve agrees with the total number of dendrites in all taste organs of each lobe.     

Cuticular sensory receptors on the antenna and maxillary palp of a fly larva, Nephrotoma suturalis (Diptera: Tipulidae)

Abstract. The apex of the larval antenna of the crane fly Nephrotoma suturalis has 6 cuticular sensilla that stained intensely black with silver nitrate, which indicates their porosity. The large sensory cone is innervated by 14 neurons and the 3 small, smooth surfaced, conical pegs have 4 neurons each. The small and large cylindrical sensilla with their smooth walls and pleated apices are innervated by 4 and 6 nerve cells, respectively. The 15 sensilla on the apex of the maxillary palp are all stained by silver nitrate. These sensilla are of five types: 7 type A sensilla with a smooth surface, a distinct apical pore, and 3 or 4 neurons; 2 type B sensilla with a smooth surface, many pores, and 5 neurons; 1 type C sensillum with a grooved surface, a large apical pore, smaller pores in the grooves, and 6 neurons; 3 type D sensilla with a smooth surface, a grooved apex that is elongated into a projection, and 4 neurons; 2 type E sensilla with many pores covering the surface, leaf-like appearance, and 4 neurons. The number and types of sensilla are similar to those in other nematocerous larvae, but in the many different forms of sensilla and the structure of the sensory cone, these tipulid larvae differ greatly from other larvae of lower Diptera.